Crystallization Kinetics of Mechanically Alloyed Al₈₀Fe₂₀ Amorphous Powder

Journal of Science & Technology 119 (2017) 066-070
66
Crystallization Kinetics of Mechanically Alloyed Al80Fe20
Amorphous Powder
Nguyen Thi Hoang Oanh, Tran Quoc Lap, Pham Ngoc Dieu Quynh, Le Hong Thang,
Nguyen Thi Anh Nguyet, Pham Ngoc Huyen, Nguyen Hoang Viet*
Hanoi University of Science and Technology – No. 1, Dai Co Viet Str., Hai Ba Trung, Ha Noi, Viet Nam
Received: June 15, 2016; accepted: June 9, 2017
Abstract
The crystallization kinetics of an Al80Fe20 amorphous powder alloy were investigated by thermal analysis.
Crystallization of amorphous Al80Fe20 during continuous heating undergoes four stages. The first-stage
crystallization leads to the formation of fcc-Al from amorphous matrix. The next stages are the
decomposition of the residual amorphous phase into several intermetallic compounds. The activation
energies of the alloy were calculated from differential scanning calorimetry data using the Kissinger, Ozawa
and Augis–Bennett models. The non-isothermal crystallization kinetics are analyzed by Johnson-Mehl-
Avrami equation. The value of the Avrami index indicated that the crystallization is interface - controlled
growth.
Keywords: amorphous alloys, mechanical alloying, crystallization kinetics, Avrami exponent
1. Introduction*
Al-rich metallic glasses have generated
considerable research interest because of the excellent
mechanical and chemical properties. Tensile strength
of Al-based amorphous alloys is 2-5 times higher
than their conventional crystalline counterparts [1-3].
Their high tensile strength can be further enhanced if
fcc-Al nano-particles are homogeneously dispersed
within a certain size and fraction range through
primary crystallization [4, 5]. One of the critical
aspects of their applications is thermal stability, as the
amorphous state is a non-equilibrium phase which
irreversibly crystallizes upon heating. The
crystallization kinetics are very important for the
development of amorphous alloys and nanocrystalline
materials, the properties of which are strongly
affected by the crystallization process. Therefore, the
crystallization kinetics of amorphous alloys have
been studied extensively. Controlling the
microstructure development from the glassy
precursors requires detailed understanding of the
specific mechanisms influencing structural
transformations. Moreover, crystallization studies are
essential for the proper choice of the consolidation
parameters in order to maximize densification and, at
the same time, retaining the desired microstructure [6,
7].
Differential scanning calorimetry (DSC)
technique allows a rapid and precise determination of
crystallization temperatures of amorphous materials.
* Corresponding author: Tel.: (+84) 904.777.570
Email: viet.nguyenhoang@hust.edu.vn
DSC has also led to the study of the crystallization
kinetics by so-called non-isothermal methods.
Several reports on the successful formation of
an amorphous phase through MA have been
published for Al80Fe20 amorphous alloy [2, 8-10]. But
there is a lack of studies regarding the crystallization
kinetics of Al80Fe20 amorphous alloy.
In this study, the thermal stability as well as the
crystallization kinetics of the mechanically alloyed
Al80Fe20 amorphous powder has been investigated
using DSC in non-isothermal modes. The value of the
Avrami index is calculated by Johnson-Mehl-Avrami
equation to determine crystallization mechanism of
Al80Fe20 amorphous powder.
2. Experimental
Al80Fe20 amorphous alloy powder was prepared
via mechanical alloying process after 60h of milling
(more details in [11]). The structure of the as-
received samples was confirmed by XRD
measurements using RIGAKU RINT-2000 with
CuKα (λ=1.5405Å) radiation. Morphology of the
amorphous powder samples was observed by a field
emission scanning electron microscope (FE-SEM).
The crystallization kinetic of the powders was
evaluated by non-isothermal DSC under a continuous
flow of Ar gas (70 mL/min) at heating rates of 5, 10,
20 and 40 K/min using NETZSCH STA 409C, where
platinum cups were used as containers.
3. Results and disscution
Fig. 1 shows the XRD pattern of Al80Fe20
powder mixture presented a fully amorphous
structure after 60 hours of milling.
Journal of Science & Technology 119 (2017) 066-070
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Fig. 1. X-ray diffraction patterns of Al80Fe20
amorphous powder.
Fig. 2. FE-SEM image of Al80Fe20 amorphous
powder after 60h of milling.
Fig. 3. DSC curves of Al80Fe20 amorphous powder at
various heating rates.
Fig. 2 illustrates the SEM/EDS observation for
as-received Al80Fe20 amorphous powder. It can be
seen that fine powder particles, the particle size
mostly below 15 µm, were agglomerated to form
larger particles
Fig. 3 presents the DSC diagram for the Al80Fe20
amorphous powder as a function of temperature taken
at different heating rates. As can be seen, this powder
has four crystallization peaks, which means that
powder undergoes four crystallization stages.
Moreover, increasing the heating rate from 5 to 40
oC/min caused all position of the exothermic
crystallization peaks shift to higher temperatures. The
peak temperature (Tp) values at different heating rates
are summarized in Table 1.
Table 1. Characteristic temperature at crystallization
peaks of Al80Fe20 powder at different heating rates
Heating rate,
K/min
Tp1,
°C
Tp2,
°C
Tp3,
°C
Tp4,
°C
5 360.9 412.0 486.0 576,.9
10 366.1 424.0 496.5 587.6
20 371.7 438.2 506.8 596.4
40 373.6 445.9 512.5 601.3
Similar observation for the temperature peak for
the first crystallization peak of those amorphous
samples were made by F. Zhou [8] with Tp1 about 400
oC. These amorphous alloys have crystallization
temperature range from 300 oC to 640 oC by F. Zhou
and from 350 oC to 630 oC in this study.
The activation energy of the crystallization
process gives important information regarding the
thermal stability of the sample. It can be evaluated
from constant-rate heating DSC curves taken at
different heating rates using the Kissinger Ozawa and
Augis-Bennett equations, as given by equation (1),
(2), (3), respectively: [12]
2ln
a
pp
E
const
RTT
 
(1)
ln( ) a
p
E
const
RT
 (2)
ln a
p o p
E
const
T T RT
 
(3)
where β is the heating rate, Tp is the temperature
at the exothermal peak, R is the gas constant and Ea is
the activation energy of crystallization. Figure 4-6
show that Kissinger plot ln(β/Tp2) versus 1000/Tp,
Ozawa plot ln(β) versus 1000/Tp, Augis-Bennett plot
ln(β/Tp-To) versus 1000/Tp, which yields straight lines
with a good fit, respectively. Table 2 presents results
of the activation energy calculated through three
methods.
Table 2. Activation energy (Ea [kJ/mol]) of Al80Fe20
amorphous powder for the crystallization stages
determined via three methods
Methods
Active Energy, kJ/mol
Peak 1 Peak 2 Peak 3 Peak 4
Kissinger 510.1 230.2 362.6 493.2
Ozawa 520.7 241.8 375.4 507.6
Augis-Bennett 515.4 236.0 369.0 500.4
5µm
Journal of Science & Technology 119 (2017) 066-070
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Fig. 4. Kissinger plots of the Al80Fe20 amorphous
powder.
Fig. 5. Ozawa plots of the Al80Fe20 amorphous
powder.
Fig. 6. Augis-Bennett plots of the Al80Fe20
amorphous powder.
It can be seen, the values of the activation
energies calculated from three models are
approximate. Therefore, we can use one of the three
methods to calculate the activation energy.
The Avrami index (n) gives detailed information
on the nucleation and growth mechanism of new
crystalline grains during the phase transition, which
can be obtained by Johnson-Mehl-Avrami (JMA)
equation: [12]
( ) 1 nkx t e  
(4)
where x is the crystallization volume fraction at
time t, n is the Avrami exponent and k is the reaction
rate constant related to absolute temperature
described by Arrhenius equation:
aE
RT
ok k e
 
(5)
where is a constant, is the activation
energy, R is the gas constant and T is the absolute
temperature.
There are 2 methods to determine the Avrami
parameter. The first method was proposed by Ozawa.
We have:
ln( ln(1 ))
ln T
d x
n
d 
(6)
The value of x at any selected T is calculated
from the ratio of the partial area of the crystallization
peak at the selected temperature T to the total area of
the exothermic peak. Fig. 7 shows diagram of
crystallized volume fraction for Al80Fe20 amorphous
powder.
Fig. 7. Crystallized volume fraction x for Al80Fe20
powder at different heating rates.
Combining equation (6) and plot (7), at any
fixed temperature, we can consider the Avrami
parameter to be 0.91 in the first crystallization event.
The second method to calculate Avrami
parameter is through the activation energy calculated
by Kissinger method, as following
Journal of Science & Technology 119 (2017) 066-070
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ln( ln(1 ))( )
1
x
R x
n x
E
T
   
(7)
The crystallized volume fraction is also
determined by measuring the corresponding partial
area of the exothermic peak. Plotting ln[-ln(1-x)]
versus ln(1/T) with x between the range of 15% to
85% of transformed fractions, the JMA plots at
different heating rates are obtained as in Fig. 8.
Fig. 8. JMA plots for 1st crystallization peaks of
Al80Fe20 amorphous alloys at different heating rates.
The Avrami index was obtained by the slopes of
these plots. The Avrami index (n) is 0.80 in the first
crystallization process. According to calculated
Avrami index calculated by 2 methods is approximate
to 1. The Avrami index usually between 1 and 4 if the
growth of the crystal is diffusion controlled. With n
less than 1, the crystal growth has been shown to be
interface controlled [13]. A low value of n has also
been reported by other investigators in the primary
crystallization of amorphous alloys. This value
suggesting that the transformation in this stage is
interface-controlled growth [14].
In order to determine the products of
crystallization, milled powders were annealed in the
DSC by heating at 20 °C/min to temperature in the
range of 413 and 670 °C, coressponding to the end
temperatures of four crystallization reactions. Fig. 9
shows XRD spectra from the amorphous Al80Fe20
alloy after heat treatment at different temperatures.
After heating to 413 °C, the amorphous alloy began
to crystallize into fcc-Al phase and remain
amorphous phase. After increase heating temperature
to 468 °C intermetallic phases of Al13Fe4, Al3Fe and
Al6Fe can be detected from XRD pattern in Fig. 8 (b).
At higher temperature of 535 °C cleary diffraction
peaks of Al13Fe4 and Al6Fe phases can be seen Fig. 8
(c). At the final heating temperature of 670 °C, no
amorphous phase can be retained, phases of fcc-Al
and Al13Fe4 can be obtained. Similar observation
regarding products of structural changes for the
amorphous alloy were made by F. Zhou et al. [8],
and M. Krasnowski [2].
Fig. 9. XRD patterns from amorphous Al80Fe20 alloy
after heat treatment at temperatures at (a) 413, (b)
468, (c) 535 and (d) 670 °C.
4. Conclusion
Crystallization kinetics of mechanically alloyed
Al80Fe20 amorphous powder have been investigated
using DSC in non-isothermal modes. The
crystallization behavior of amorphous powder occurs
in four stages in the temperature range of 350 and 630
oC. The primary phase of fcc Al together with
maintaining amorphous phase in the first
crystallization event followed by formation of
Al13Fe4, Al3Fe and Al6Fe intermetallic phases in the
second crystallization event. At the higher
crystallization temperature in the third crystallization
stage, intermetallic phases of Al13Fe4 and Al6Fe
occurred. In the final exothermic event, phases of fcc-
Al, Al13Fe4 and AlFe3 can be realized. The values of
activation energy calculated from three methods
Kissinger, Ozawa and Augis-Bennett are almost
same. The Avrami exponent is less than 1 for the first
crystallization peak, suggesting that the
transformation was interface - controlled growth.
Acknowledgments
This research is funded by Vietnam National
Foundation for Science and Technology
Development (NAFOSTED) under grant number
103.02-2012.19.
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